Re-Adding BlendMode Presenter definition.

Re-Adding BlendMode Presenter Definition.

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/Presenters/MaterialGraphPresenter.cs

             typeMapper[typeof(LevelNode)] = typeof(LevelNodePresenter);
             typeMapper[typeof(ConstantsNode)] = typeof(ConstantsNodePresenter);
             typeMapper[typeof(SwizzleNode)] = typeof(SwizzleNodePresenter);
+			typeMapper[typeof(BlendModeNode)] = typeof(BlendModeNodePresenter);
         }
     }
 }